This is the first of a three-part question on the basilisk plot in Harry Potter and the Chamber of Secrets.

Out of all the Harry Potter books, Chamber of Secrets is the most problematic for me as far as continuity and plot goes. I have issues with the whole basilisk plot. I have a set of three questions regarding Chamber of Secrets that I'm looking for CANON COMPLIANT explanations for. By "canon compliant", I mean within the spirit of canon, answers directly from the book(s), or quotes from J.K. Rowling.¹

In Chamber of Secrets the basilisk moves about the castle through
the plumbing. However, if Hogwarts was built 1000+ years ago, that
would precede indoor plumbing. How can this be explained, that the castle has original indoor plumbing? How could the Founders have anticipated this innovation?

The basilisk is described as very large: "as thick as an
oak trunk" (CoS - page 318 - US Hardcover) and "able to grow up to fifty feet long" (FBAWTFT - page 3 - Scholastic) Would a snake this large be able to feasibly fit through standard-sized pipes used for indoor plumbing, even pipes sized to accommodate Hogwarts?

At the point indoor plumbing was installed in Hogwarts, how is it
that the entrance to the Chamber of Secrets wasn't obvious and easily
discovered by those installing the indoor plumbing?

How could sinks be installed above the entrance, complete with a faucet marked with a serpent, indicating the entrance, without anyone knowing the significance of the serpent on the faucet?

How could Hogwarts have feasibly accommodated a basilisk 800-1000+ years ago, presumably without indoor plumbing?

¹ I find the Harry Potter Wiki to be inconsistent and oftentimes incorrect. I am not looking for any answer(s) sourced from the HP Wiki unless Wiki itself backs that precise fact with explicit canon quote. Just an FYI.

I don't believe there's any mention of the basilisk moving around the school prior to the first opening of the chamber of secrets, which was only about 50 years or so earlier (I'm still kinda sketchy on exact timelines).
– morganpdxMar 13 '12 at 0:48

5 Answers
5

The Chamber was NOT designed for indoor plumbing. If you recall, once they fell through the pipe, they were walking in the tunnel, with no pipes mentioned. The Chamber was not a "place where plumbing goes". It was more of a catacombs/hidden chambers, under the lake (presumably as per Ron's guess - "Under the lake, probably" was the exact quote), which was a part of architecture and castles since Well Before Hogwarts (e.g. Romans).

The plumbing was merely an easier way to get from the castle proper to the catacombs - and more likely than not, existed in some primitive form in original Hogwarts (e.g. a water spring for hand washing) even if not a bathroom.

The trickier question was whether the entrance from the bathroom dated from Slytherin's time, as it looked to be more modern. But for a wizard of Slytherin's accomplishments, making a spell creating an entrance which would "conform" itself to the current surroundings of the room was surely NOT out of the question - heck, they created Room of Requirement! A mimicking entrance is incomparably simpler!

Now, a much more difficult question is, if the Serpent used the same pipe that Harry and Ron fell through, how was it able to climb UP? It sounded like an extremely steep, nearly vertical incline.

As far as fitting into plumbing - no canon answer. But THIS! IS! HOGWARTS! </King_Leonidas>. The stairways reconfigure themselves. Why not plumbing?

I sort of assumed that the Basilisk climbed up the pipe sort of like arboreal tree snakes are able to maneuver themselves up and through trees. My biggest issue comes from the size of the creature!
– Meg CoatesMar 13 '12 at 16:14

"Look at the SIZE of that thing!" - SW.
– DVK-on-Ahch-ToMar 13 '12 at 16:28

3

I don't think a snake would have any problem climbing up even a completely vertical pipe. This spring I saw a 4,5 meter long snake go straight up 2,5 meters against a wall like it was nothing. In a pipe the snake would be able to support itself in any direction it wanted, making the task of climbing easier.
– AifosNov 11 '12 at 2:17

An answer comes from a Pottermore update which describes the history of the Chamber of Secrets (transcript), along with some speculation of my own.

Here’s the relevant passage:

When first created, the Chamber was accessed through a concealed trapdoor and a series of magical tunnels. However, when Hogwarts’ plumbing became more elaborate in the eighteenth century (this was a rare instance of wizards copying Muggles, because hitherto they simply relieved themselves wherever they stood, and vanished the evidence), the entrance to the Chamber was threatened, being located on the site of a proposed bathroom. The presence in school at the time of a student called Corvinus Gaunt – direct descendant of Slytherin, and antecedent of Tom Riddle – explains how the simple trapdoor was secretly protected, so that those who knew how could still access the entrance to the Chamber even after newfangled plumbing had been placed on top of it.

(In spoiler text for those who want to work through Pottermore themselves; spoilers in the discussion below.)

So here are my answers to your specific questions:

Indoor plumbing in the original castle. They didn’t need to explain indoor plumbing, because they didn’t include it with the original castle. Instead, it moved through “a series of magical tunnels”.

Just as the stairs can reconfigure themselves, I’d expect the pipes can reconfigure themselves as well. If they retract when the basilisk wasn’t active, then it would make any search for the Chamber near impossible unless you stumbled upon the Chamber itself.

Size of the pipes. The Room of Requirement can change its size as required, so presumably these tunnels are enchanted with similar magic. When the indoor plumbing was installed, the two systems could just be interconnected.

You can imagine that the indoor plumbing might have been similarly enchanted to change size to accommodate the required flow, or to be self-cleaning. This would make it harder to distinguish the tunnels used by the basilisk.

The introduction of indoor plumbing. There are no details on how Corvinus Gaunt concealed the entrance, but I can guess.

Just as Tom Riddle presented an acceptable face to the school, and hid his role in the opening of the Chamber, so I think the Gaunts could have presented a somewhat acceptable face (as teenagers, at least). This might let Corvinus influence the construction of the bathroom, and ensure nobody asked any awkward questions.

If not that, then memory charms and coercion probably go a long way.

The marking on the sink. Again, no details, but speculation.

We learn from Myrtle that this sink is permanently broken. That would surely attract attention from the Hogwarts maintenance staff, and eventually somebody would notice the snake emblem on the tap (people are oblivious, but not that oblivious).

So I think it must have been enchanted to avoid scrutiny, similar to a Muggle-repelling charm. Just like the Room of Requirement, perhaps it’s concealed so that only somebody who knows to look for it can find it. Corvinus would have supervised its installation, so he could have added the appropriate protections.

Remember that the Chamber of Secret (and most importantly the basilisk) was a vengeance device. Slytherin didn't plan any movement mean for the Basilisk because when he left Hogwart, I guess the basilisk was still an egg. He designed this room as a tool for the use of a future heir of his. I strongly think that the basilisk was not put there as a "splinter cell" murderer, but as a war weapon.

Riddle did release the basilisk once, but before that the basilisk was probably in a magic sleep state, thus not needing to move.

An interesting theory. I'm trying to remember if JKR said anything about whether the Basilisk was alive during Slytherin's time. I'll have to check what she said about the Chamber of Secrets. b_jonas has a great idea as to where you can leave this info in a second question, too. :)
– SlytherincessMay 5 '13 at 14:57

As stated above, indoor piping is not something new, it was found Ancient Carthago. Now, Hogwarts is a magical castle so it wouldn't be surprising if it looked very different in the 1990's than it did 1000 years ago.

The tunnel is mostly sinuous so the Basilisk would have no problem slythering up, but because the no other pipe is large enough for the Basilisk (Harry noticied as the slided toward the chamber) the there has to be a secret door for the basilsk to get access to the indoor piping network, for instance on a wall opposite to the entrance of the bathroom, a network that would have been exclusively designed by Salazar for the Basilisk. The chamber would be the "ammuntion storage" the piping network the barrel of the gun, and the Basilisk the bullet.